Metal-organic frameworks (MOFs) is an emerging class of porous solid-state materials with significant contributions in numerous application areas including, but not limited to, catalysis, separations, gas storage, and drug delivery. In this typical class of periodic solids, there has been great progress toward design, due largely to the ability to target specific [molecular] building blocks with given geometry and directionality (e.g. squares, tetrahedra, etc.) prior to the assembly process.
Recently, there has been an increased effort to generate 3D porous MOFs via “pillaring” these layered MOFs. One approach, referred to here as axial-to-axial (A-A) pillaring, takes additional advantage of the auxiliary axial metal sites of a paddlewheel configuration, which are typically occupied by terminal ligands like water or pyridine. In this case, pairs of terminal ligands are replaced by a ditopic (bridging) ligand, that can coordinate the axial positions of two dimer units from neighboring layers resulting in bridged MOF layers based on six-connected dimer units.
Another pillaring method involves what we call ligand-to-ligand (L-L) pillaring, where specific ligands are selected to simultaneously contain two bridging ligand moieties that pillar adjacent layers through the covalent linkage of the tetracarboxylate ligand. When the 4-connected ligand coordinates to form the 4-connected paddlewheel MBB, the resulting 3D MOF is based on a (4,4)-connected topology (e.g., nbo-MOFs).
However, alternative pillaring strategies are needed to form other types of layered MOFs.